The articulation of the tibia and femur of a normal human knee joint is not perfectly straight, but is bent outward or away from the centerline of the body. This condition is known as valgus, with normal considered to be at approximately six degrees. This creates a mechanical axis extending from the head of the femur, through the center of the knee joint, to the center of the ankle joint. Assuming the mechanical axis of the leg is within normal parameters, the loading patterns on the leg, which can be great, will be properly distributed.
Several conditions can lead to an alteration of the normal mechanical axis of a leg. Degenerative osteoarthritis can sometimes cause a condition in long human bones which causes the bone to change shape. Instead of being relatively straight, the bone becomes curved or misshapen over a period of time. This can be either an outward direction (valgus), in an inward direction (varus), or compound shapes. Most frequently, the valgus or varus condition occurs below the articular surface of the tibia, affecting the load distribution on the knee joint. The result of this condition is that the mechanical axis of the leg becomes altered, with resultant load patterns that the knee joint cannot properly accomrnodate. Great pain and difficulty of movement can result, with a consequent restriction of lifestyle. A further problem associated with this condition is increased joint arthritis due to improper load distribution. In younger patients, improper bone growth can lead to similar problems, and require similar treatment. Examples of this condition are commonly known as bowleggedness (genu varum) ox knock knee (genu valgum).
Among the techniques developed tot dealing with valgus or varus conditions, one of the most successful is known as cuneiform or wedge osteotomy. Treatment requires that the angular deformity be determined by one of several methods. The desired correction angle is determined by adding the preoperative varus angle or valgus angle to the desired postoperative valgus angle. Generally, between five and thirteen degrees of valgus have been shown to result in the most successful outcome in osteotomy procedures.
In performing a cuneiform or wedge osteotomy, a wedge shaped section of bone is surgically removed to allow realignment of the bone. The wedge cut made by the surgeon should not completely sever the tibia, however. A bridge of residual bone should remain uncut, this portion serving in effect as a "hinge" for reduction of the wedge-shaped gap that remains following removal of the bone wedge. The depth of the wedge is critical in that if the wedge is too shallow, thus rendering the bridge too wide, the resulting wide fulcrum results in over stressing of the medial cortex during wedge compression, causing fracture of the bone bridge. If the wedge is too deep (the residual bone bridge is too narrow) the residual bone bridge, if not completely severed, lacks significant strength to provide medial stability to the reduced osteotomy. Reduction of the wedge-shaped gap allows realignment of the bone, and correction of the loading patterns of the leg. Bone plates will be installed to secure the reshaped bone, at least through the healing process.
Several types of devices have been developed to assist the surgeon in carrying out the osteotomy process. One type of device comprises a sequence of jigs which are affixed to the tibia in a position determined by the surgeon. This requires the drilling of holes in the tibia to anchor the jigs. A first jig contains a single slot through which the surgeon inserts the blade of an oscillating bone saw and makes a first cut. The first jig is removed and a second jig containing a series of slots corresponding to different angles is then affixed to the same position as the first jig. The surgeon inserts the oscillating bone saw blade into the desired slot and makes a second cut. The second cut should terminate at the same point as the first cut, creating a clean apex. Ideally, this allows a section of bone to remain integrated. Careful compression of the resected area reshapes the bone, and is secured by the placement of an osteotomy bone plate.
Problems in using jig based devices include the possibility of inaccurate estimation of the resection surfaces, resulting in an insufficient area of residual bone, or even of a severing of the upper part of the tibia. Also, the two cuts may not be aligned properly, resulting in an unclean cut which could interfere with the compression and mating process, and also affect the healing process. By their nature, jigs are rigid devices which are unable to adjust to the varied contours of an individual patient's anatomy. Thus, using such devices is often a compromise, and can lead to less than ideal results.
Other devices are also known which provide varying degrees of adjustability to allow the surgeon to select a predetermined desired angle. These devices typically require the surgeon to make an unaided first cut. The device includes a flat, blunt blade which is inserted into the first cut. The surgeon then adjusts the angle of the saw blade guide on the device to create a properly angled wedge to be excised from the bone. Problems with such devices include a lack of precision, with the possibility of locating the wedge apex too shallow or too deep and producing an improperly sized or angled wedge.
What is clearly needed, therefore, is a method and instrumentation for performing osteotomy procedures which allows greater accuracy and improved precision in determining the location of the resection surfaces. Also highly desirable would be a system allowing a greater degree of flexibility in positioning the device in a coronal plane or angle it away from the coronal plane. This would permit an osteotomy procedure to be biased in a posterior or anterior slope, if required. Finally, an osteototmy system permitting resectioning procedures on bones, other than the upper tibial region would be highly desirable.